1. Field of the Invention
The present invention relates to a corona discharge method and more specifically to the improvement in the method of transferring toner images and separating the transfer material loaded with toner particles from the image receptor, especially in the electrophotographic reproducing apparatus which uses a corona discharger.
2. Description of the Prior Art
In one of the methods available for separating the transfer material in the conventional reproducing apparatuses, an adhesive transfer material is placed in contact under pressure with the image receptor to transfer the toner image from the image receptor to the transfer material and then the transfer material is separated from the image receptor by a blade. In other conventional method, a voltage with a polarity opposite to the toner charge on the image receptor is applied by the electrode roller or corona discharger to the back of the transfer material to transfer the toner image from the image receptor to the transfer material and then the transfer material is separated from the image receptor by a means similar to the above or the corona discharger. The latter method employing the corona discharger for the image transfer and separation is widely used because of the advantages that it does not need special transfer material which has adhesiveness; that there is no possibility of the toner image being pressed flat as when the adhesion image transfer or electrode rollers are used; that relatively high transfer efficiency is obtained; that the image receptor is free from being damaged as when the toner image is separated by the separation blade; and that the structure of the device is simple.
In the conventional toner image transfer and transfer material separation method using a corona discharger, as shown in FIG. 1, two separate corona dischargers 1, 1' are used for the image transfer and the transfer material separation respectively. DC and AC corona ion flows are obtained by applying a DC voltage to the discharge wire 2 of the corona discharger 1, applying an AC voltage to the discharger wire 2' of the corona discharger 1' and applying a DC voltage to the cover plates 3, 3' or scorotron grid not shown, or grounding the cover plates 3, 3'.
This method, though advantageous when compared to other methods, has the following drawbacks. As the charge holding performance of the transfer material P decreases due to humidity, the transfer efficiency may also deteriorate. When the DC corona ion flow is intensified to improve the transfer efficiency, the charge of corona ion flow may penetrate through the transfer material P into the toner 5 at the transfer area A, charging the toner 5 to the same polarity with the charge of corona ion flow with the result that the toner 5 is repulsed from the transfer material P resulting in the further reduction in the transfer efficiency or disturbance in the image. The strongly charged transfer material P may attract a part of toner from the image receptor 4 at the pretransfer area B causing image disturbance. Further, since the corona ion flow is symmetrical with respect to the direction in which the transfer material is moving, the reduction in the transfer efficiency and the image disturbance may also result at the separation area C succeeding the transfer area A due to similar reasons cited for the transfer area A and pretransfer area B. To avoid these problems, the discharging conditions of the corona dischargers 1, 1' are necessarily restricted to a very narrow range.